The UK government’s Environment Agency has published a regulatory position statement on the Internet which tells you the rules about scattering pet ashes in the UK.
Although you usually need a permit to spread or bury ashes in the UK, provided you comply with the conditions you don’t need to apply for a one. It clearly puts the onus on the cat owner to behave properly because if you are in breach of the rules no one is going to know. I’m not suggesting that you do break the rules but these rules are all but impossible to enforce.
The “regulatory position statement” covers the burial and scattering of ashes from the cremation of a pet. You should ensure that the scattering is carried out at a pet cemetery registered with the Animal and Plant Health Agency and the ash is buried at a density which does not exceed average of 20 kg/m² and that the burial or scattering of the ash is not carried out on grazing land and further you must make sure that the scattering does not endanger human health or the environment. I find these criteria a bit strange because it states that you have to scatter the ashes at a pet cemetery and then refers to grazing land. Why mention grazing land? Are there some pet cemeteries doubling up as grazing land?
You cannot cause a risk to animals, plants, soil, air or water or causing nuisance through noise or odours or adversely the countryside or places of special interest.
Provided you meet all these criteria you do not need to apply to the Environment Agency for a bespoke permit but if you cannot comply you will have to seek a permit to bury or spread your pet’s ashes.
Comment: I read somewhere on the Internet that the UK was quite relaxed about spreading pet ashes but having read the Environment Agency’s webpage I have changed my mind because there are restrictions and they are quite extensive and frankly confusing. However, as I said earlier, it would be extremely difficult for the Environment Agency to take enforcement action against if you hadn’t applied for a permit or had done something in breach of these rules. That said, please comply!